Use ExceptT

See merge request !15

Update dependencies, take advantage of some improvements

See merge request !14

As a bonus, runGBS authenticates the server for us.

tahoe-chk in particular has a number of API incompatibilities, so also deal
with those.

Fix up metadata

See merge request !13

fetch all the shares at the same time

Closes #11

See merge request !12

Try to ask for the right share number

Closes #10

See merge request !9

expose DirectoryDownloadError for proper error handling in client applications

See merge request !10

And try more than one if necessary / possible

Add `downloadDirectory` function!

See merge request !8

I do not know why.